This invention relates to an apparatus for use in conjunction with electrical equipment, to monitor the ground circuit connection to such equipment and to disable the voltage applied to the equipment whenever the ground circuit becomes opened. In one embodiment of the invention the apparatus senses the presence of a voltage between the alternating current power line and ground, and disables a circuit breaker connection controlling power to the equipment whenever a voltage cannot be sensed between the power line and ground. In another embodiment the apparatus also detects the quality of the ground circuit, and disables power to the equipment to which it is connected whenever the ground circuit resistance increases over a predetermined level.
In the past, most circuit monitoring apparatus has been designed for detecting a ground fault condition, wherein a short circuit condition exists between ground and some voltage point in the circuit being controlled. Prior art monitoring circuits have been devised for monitoring the degree of unbalance of current flow in power lines to thereby detect when one or more of these lines is losing current to a ground connection. Other circuits have utilized a pilot wire to monitor ground circuits, disabling the voltage power whenever either the pilot line or the ground line becomes opened.
All of these prior art circuits, as well as the present invention, function to provide protection for equipment and operators whenever a fault condition arises. The present invention is concerned primarily with operator protection, to disable the voltage applied to equipment the operator is handling whenever an open ground circuit is detected. The need for this operator protection is important, for if an open ground circuit occurs while an operator is handling electrical equipment the operator may suddenly become the only circuit path to ground and potentially fatal currents may flow through the operator's body.
Modern alternating current power supplies utilize three wires for delivering voltage and circuit ground to equipment and appliances connected thereto. A typical household 110 volt AC power receptacle has three sockets, one for delivering "hot" AC voltage, the second for delivering the AC voltage "neutral", and the third for providing an absolute circuit ground. When an equipment or appliance is plugged into such a receptacle, assuming it has a three-wire connecting cord, the "hot" line is connected to one side of the voltage circuits and the "neutral" line is connected to the other side of the voltage circuits, and frequently also to an internal equipment ground. The third or ground wire connection is made to the equipment chassis or case to assure that such chassis or case is maintained at an absolute ground potential. If the operator utilizes an extension cord having only two wires, or otherwise disables the third ground connection, the safety provided by the absolute ground connection to equipment chassis or case is eliminated, and a potential voltage shock situation exists. In this situation the present invention will disable the power applied to the equipment until a proper ground connection is made.